Windermere Group (geology)
Encyclopedia
The Windermere Supergroup is a geological unit formed during the Ordovician to Silurian periods ~, and exposed in the north west of England
, including the Pennines
and correlates along its strike, in the Isle of Man
and Ireland, and down-dip in the Southern Uplands and Welsh Borderlands. It underlies much of north England's younger cover, extending south to East Anglia. It was formed as a foreland basin
, in a similar setting to the modern Ganga basin
, fronting the continent of Avalonia
as the remains of the attached Iapetus ocean
subducted under Laurentia
.
The supergroup comprises the Coniston group
of turbiditic limestones, and the overlying series of shale
s, grit
s and greywacke
s. Compression from the south east during the later Acadian orogeny (probably caused by the closure of the Rheic ocean) buckled the strata into anticlines and synclines and caused slaty
cleavage
in some sediment beds.
, a sequence that formed on the Avalonia
n continental margin, composed mainly of turbidites. Between the two, the Borrowdale Volcanic Group consists of tuffs erupted by an underlying calc-alkaline
volcanic arc, active during the subduction of the Iapetus ocean crust.
The Windermere basin was formed by flexure. Prior to its formation, the Southern Uplands accretionary prism
, flanking the edge of the Laurentian continent, was advancing towards Avalonia. The load of the mountains formed during this collision weighed down the Avalonian plate, causing the development of accommodation space.
transitions, with black shales corresponding to transgressions - these may have helped to mitigate a runaway greenhouse effect
.
The rate of sediment accumulation accelerated with time; it held fairly steady at a low ~50 metres per million years (m/Ma) until the Wenlock (mid Silurian), when it increased greatly, eventually reaching over 1000 m/Ma when the record is terminated by erosion in the Pridoli (terminal Silurian). This sudden upturn in deposition rate is a result of the increasing proximity of the Avalon mountain belt, which started to depress the plate from the Ordovician, but was not close enough to increase the sedimentary input until the Silurian. The latest phase of sedimentation reflected a change in the basin's state. Instead of being underfilled, and trapping all sediment that flowed into it, it became overfilled. This was reflected by a shallowing of water depth, as the basin silted up. This culminated with a transition to terrestrial conditions in the Přídolí.
from a section across the Windermere Supergroup permits an estimate to be made of its maximum burial depth. Deposits at the surface today were once covered with 5-6 km of sediment; some of this would belong to faulted Windermere deposits, but it is postulated that the bulk of it was Old Red Sandstone
, including molasse
deposits laid down by alluvial fans on the flanks of the mountain belt, and probably lower energy fluvial (river) or aeolian (dune) deposits.
, to an overfilled one (stage 3). The orogenic wedge then provides a significant source of molasse
sediments, with turbidites and deltas prograding through the basin. The basin is eventually filled, and covered with fluvial and alluvial molasse (stage 4).
. The deeper water deposits of stage three are represented by the Stockdale and Tranearth groups, which display a steady deepening, as expected of sediments deposited in an underfilled basin, with a sedimentation rate high enough to preserve annual variation in places. (This signal is complicated by an overprinted signature of eustatic glaciations.) The end of stage three is represented by the Coniston Group, a series of sandy turbidites, with sediment supply from the north east (and controlled by basement faulting). The group is subdivided into formations, which each represent a turbidite lobe, and are separated by anoxic background sedimentation. The Kendal Group covers the transition into stage four, with pronounced shallowing up; turbidites become thinner-bedded, and anoxic hemipelagics give way to oxygenated sediments, with storm beds becoming more and more common, and intertidal sediments topping the group. The missing Old Red Sandstone mentioned above formed the molasse sediments of stage 4.
Throughout the Silurian, until the beginning of Old Red Sandstone deposition, sedimentation rate increases steadily, reaching a peak of 1 mm a-1.
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
, including the Pennines
Pennines
The Pennines are a low-rising mountain range, separating the North West of England from Yorkshire and the North East.Often described as the "backbone of England", they form a more-or-less continuous range stretching from the Peak District in Derbyshire, around the northern and eastern edges of...
and correlates along its strike, in the Isle of Man
Isle of Man
The Isle of Man , otherwise known simply as Mann , is a self-governing British Crown Dependency, located in the Irish Sea between the islands of Great Britain and Ireland, within the British Isles. The head of state is Queen Elizabeth II, who holds the title of Lord of Mann. The Lord of Mann is...
and Ireland, and down-dip in the Southern Uplands and Welsh Borderlands. It underlies much of north England's younger cover, extending south to East Anglia. It was formed as a foreland basin
Foreland basin
A foreland basin is a depression that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure...
, in a similar setting to the modern Ganga basin
Ganga basin
The Ganges basin is a part of the composite Ganges-Brahmaputra-Meghna basin draining 1,086,000 square kilometres in Tibet, Nepal, India and Bangladesh. To the north, the Himalaya or lower parallel ranges beyond form the Ganges-Brahmaputra divide. On the west the Ganges Basin borders the Indus...
, fronting the continent of Avalonia
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...
as the remains of the attached Iapetus ocean
Iapetus Ocean
The Iapetus Ocean was an ocean that existed in the Neoproterozoic and Paleozoic eras of the geologic timescale . The Iapetus Ocean was situated in the southern hemisphere, between the paleocontinents of Laurentia, Baltica and Avalonia...
subducted under Laurentia
Laurentia
Laurentia is a large area of continental craton, which forms the ancient geological core of the North American continent...
.
The supergroup comprises the Coniston group
Coniston Limestone
Coniston Limestone is the sedimentary rock formation around Coniston in the English Lake District, and forms part of the Windermere Supergroup. It is late Ordovician or possibly early Silurian in age and rests unconformably upon the Borrowdale Volcanic Group of rocks, which subsided beneath the...
of turbiditic limestones, and the overlying series of shale
Shale
Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals, especially quartz and calcite. The ratio of clay to other minerals is variable. Shale is characterized by breaks along thin laminae or parallel layering...
s, grit
Grit
Grit may refer to:* GRIT , also known as Arhgap32 or PX-RICS* Grit , a U.S. periodical founded as a newspaper in 1882* Grit , by Celtic fusion musician Martyn Bennett* Grit , by Norwegian rock band Madrugada...
s and greywacke
Greywacke
Greywacke or Graywacke is a variety of sandstone generally characterized by its hardness, dark color, and poorly sorted angular grains of quartz, feldspar, and small rock fragments or lithic fragments set in a compact, clay-fine matrix. It is a texturally immature sedimentary rock generally found...
s. Compression from the south east during the later Acadian orogeny (probably caused by the closure of the Rheic ocean) buckled the strata into anticlines and synclines and caused slaty
Slate
Slate is a fine-grained, foliated, homogeneous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade regional metamorphism. The result is a foliated rock in which the foliation may not correspond to the original sedimentary layering...
cleavage
Cleavage (geology)
This article is about rock cleavage, for cleavage in minerals see Cleavage Cleavage, in structural geology and petrology, describes a type of planar rock feature that develops as a result of deformation and metamorphism. The degree of deformation and metamorphism along with rock type determines the...
in some sediment beds.
Before the Windermere: Basement terranes
To the north west of the unit lies the Cambro-Ordovician Skiddaw GroupSkiddaw Group
For the Skiddaw group of hills, see Skiddaw GroupThe Skiddaw Group is a group of sedimentary rock formations named after the mountain Skiddaw in the English Lake District. The rocks are Ordovician in age. They are largely mudstones and siltstones with subordinate wacke-type sandstones...
, a sequence that formed on the Avalonia
Avalonia
Avalonia was a microcontinent in the Paleozoic era. Crustal fragments of this former microcontinent underlie south-west Great Britain, and the eastern coast of North America. It is the source of many of the older rocks of Western Europe, Atlantic Canada, and parts of the coastal United States...
n continental margin, composed mainly of turbidites. Between the two, the Borrowdale Volcanic Group consists of tuffs erupted by an underlying calc-alkaline
Calc-alkaline
The calc-alkaline magma series is one of two main magma series in igneous rocks, the other magma series being the tholeiitic. A magma series is a series of compositions that describes the evolution of a mafic magma, which is high in magnesium and iron and produces basalt or gabbro, as it...
volcanic arc, active during the subduction of the Iapetus ocean crust.
The Windermere basin was formed by flexure. Prior to its formation, the Southern Uplands accretionary prism
Accretionary wedge
An accretionary wedge or accretionary prism is formed from sediments that are accreted onto the non-subducting tectonic plate at a convergent plate boundary...
, flanking the edge of the Laurentian continent, was advancing towards Avalonia. The load of the mountains formed during this collision weighed down the Avalonian plate, causing the development of accommodation space.
Sedimentation begins: Filling the basin
Sedimentation began in the Caradoc (upper Ordovician). During the Llandovery, the Stockdale Supergroup is marked by a number of oxic-anoxicAnoxic event
Oceanic anoxic events or anoxic events occur when the Earth's oceans become completely depleted of oxygen below the surface levels. Although anoxic events have not happened for millions of years, the geological record shows that they happened many times in the past. Anoxic events may have caused...
transitions, with black shales corresponding to transgressions - these may have helped to mitigate a runaway greenhouse effect
Runaway greenhouse effect
A runaway greenhouse effect is not a clearly defined term, but is understood to mean an event analogous to that which is believed to have happened in the early history of Venus, where positive feedback increased the strength of its greenhouse effect until its oceans boiled away...
.
The rate of sediment accumulation accelerated with time; it held fairly steady at a low ~50 metres per million years (m/Ma) until the Wenlock (mid Silurian), when it increased greatly, eventually reaching over 1000 m/Ma when the record is terminated by erosion in the Pridoli (terminal Silurian). This sudden upturn in deposition rate is a result of the increasing proximity of the Avalon mountain belt, which started to depress the plate from the Ordovician, but was not close enough to increase the sedimentary input until the Silurian. The latest phase of sedimentation reflected a change in the basin's state. Instead of being underfilled, and trapping all sediment that flowed into it, it became overfilled. This was reflected by a shallowing of water depth, as the basin silted up. This culminated with a transition to terrestrial conditions in the Přídolí.
Beyond the record: Postulated cover
Analysis of the clay mineral illiteIllite
Illite is a non-expanding, clay-sized, micaceous mineral. Illite is a phyllosilicate or layered alumino-silicate. Its structure is constituted by the repetition of tetrahedron – octahedron – tetrahedron layers. The interlayer space is mainly occupied by poorly hydrated potassium cations...
from a section across the Windermere Supergroup permits an estimate to be made of its maximum burial depth. Deposits at the surface today were once covered with 5-6 km of sediment; some of this would belong to faulted Windermere deposits, but it is postulated that the bulk of it was Old Red Sandstone
Old Red Sandstone
The Old Red Sandstone is a British rock formation of considerable importance to early paleontology. For convenience the short version of the term, 'ORS' is often used in literature on the subject.-Sedimentology:...
, including molasse
Molasse
The term "molasse" refers to the sandstones, shales and conglomerates formed as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse is deposited in a foreland basin, especially on top of flysch, for example that left from the rising Alps, or erosion in the Himalaya...
deposits laid down by alluvial fans on the flanks of the mountain belt, and probably lower energy fluvial (river) or aeolian (dune) deposits.
Modelling the basin's evolution
Sinclair's model of foreland basins has remained the state of the art for over a decade, and his four-stage model provides a good match for the Windermere supergroup. In the first stage, an orogenic wedge (here, the mountains of the Southern Uplands accretionary wedge) loads a passive margin, causing flexural subsidence and providing accommodation space. A "forebulge", caused by the rigidity of the crust flexing up behind the load, causes uplift and permits erosion. As the bulge moves backwards, it leaves shallow waters in its wake, which can be filled with carbonates, while hemipelagic sediments and turbidites continue to fill the deeper parts of the basin, leaving a "trinity" of facies --- this is stage 2. At a certain point, the deep water basin changes from an underfilled state, where accommodation space is created as fast as it is filled with flyschFlysch
Flysch is a sequence of sedimentary rocks that is deposited in a deep marine facies in the foreland basin of a developing orogen. Flysch is typically deposited during an early stage of the orogenesis. When the orogen evolves the foreland basin becomes shallower and molasse is deposited on top of...
, to an overfilled one (stage 3). The orogenic wedge then provides a significant source of molasse
Molasse
The term "molasse" refers to the sandstones, shales and conglomerates formed as terrestrial or shallow marine deposits in front of rising mountain chains. The molasse is deposited in a foreland basin, especially on top of flysch, for example that left from the rising Alps, or erosion in the Himalaya...
sediments, with turbidites and deltas prograding through the basin. The basin is eventually filled, and covered with fluvial and alluvial molasse (stage 4).
Matching the model
Stage 1 is hard to spot in the sedimentary record. The Dent Group, the youngest group of the supergroup, is a good match for the carbonate facies expected in the shallow waters of stages 2-3; accommodation space was created through thermal subsidenceThermal subsidence
In geology and geophysics, thermal subsidence is a mechanism of subsidence in which conductive cooling of the mantle thickens the lithosphere and causes it to decrease in elevation. This is because of thermal contraction: as mantle material cools and becomes part of the mechanically-rigid...
. The deeper water deposits of stage three are represented by the Stockdale and Tranearth groups, which display a steady deepening, as expected of sediments deposited in an underfilled basin, with a sedimentation rate high enough to preserve annual variation in places. (This signal is complicated by an overprinted signature of eustatic glaciations.) The end of stage three is represented by the Coniston Group, a series of sandy turbidites, with sediment supply from the north east (and controlled by basement faulting). The group is subdivided into formations, which each represent a turbidite lobe, and are separated by anoxic background sedimentation. The Kendal Group covers the transition into stage four, with pronounced shallowing up; turbidites become thinner-bedded, and anoxic hemipelagics give way to oxygenated sediments, with storm beds becoming more and more common, and intertidal sediments topping the group. The missing Old Red Sandstone mentioned above formed the molasse sediments of stage 4.
Throughout the Silurian, until the beginning of Old Red Sandstone deposition, sedimentation rate increases steadily, reaching a peak of 1 mm a-1.